Most medical diagnostic equipment capable of providing a three-dimensional image of an interior region of the human body is extremely bulky and costly. The space requirements and costs associated with Magnetic Resonance Imaging (MRI) equipment sometimes render it an impracticable diagnostic option, even when a three-dimensional image is required for proper diagnosis. Conventional computer-aided tomography (CAT) scanning systems, aside from being bulky and expensive, additionally require the exposure of the patient to potentially harmful radiation.
Optical methods for the detection and identification of objects embedded in scattering media, such as human tissue, are desirable because optical systems can be compact, light-weight, and relatively inexpensive while also eliminating the need for radiation from harmful portions of the electromagnetic spectrum. Optical systems are also capable of determining the chemical makeup of a structure by performing a spectral analysis, a capability which is not available from MRI or CAT imaging systems.
One problem with optical systems for medical applications is that the human body is a turbid, or scattering, medium which causes incident light to be diffusely directed away from a target object, thus obscuring the location and surface topology data from the object. A successful optical diagnostic tool for imaging objects embedded in turbid media must be able to extract useful information from the multiply-scattered light signals reflected from the embedded target.
Barbour, et al., U.S. Pat. No. 5,137,355 entitled "Method of Imaging a Random Medium," (hereinafter the "Barbour '355 patent") is incorporated by reference herein. This patent discloses a non-invasive medical imaging technique based on the measurement of scattered light in the near-infrared (NIR) region of the electromagnetic spectrum, where significant penetration into body tissues occurs. Prior to the foregoing technique, much work had been done on the optical detection of targets in turbid media, but the problem of imaging the depth or structure of an embedded object remained. The technique of the Barbour '355 patent allowed an observer to accurately detect, three-dimensionally image, and spectroscopically characterize target objects located within a turbid medium.
The technique disclosed in the Barbour '355 patent employs a multi-wavelength collimated source and a collimated receiver and performs a positional and angular scan of the scattered light from the specimen for each position of the incident beam. The technique allows the determination of an object's depth, structure, and absorptive and scattering properties within the turbid medium.
The methodology set forth in the Barbour '355 patent represents a basic description concerning how to recover images of the interior structure of highly scattering media. The only requirement is that it expects the detected signal to have undergone sufficient scattering such that every propagation is accurately described according to a particle picture (i.e. the radiation transport equation). Thus, this methodology holds for any type of energy source (e.g. electromagnetic, acoustic, particle beam) and for any source condition (i.e. DC, time resolved, or AC) in terms of assessment of the temporal characteristics of the propagating signal. While this methodology is correct in general terms, it does not speak to specific details regarding how best to perform a measurement, in particular, under non-ideal conditions.
For example, a preferred requirement for the construction of a high-quality image is the elimination of such non-ideal factors as motion artifacts, arising from unavoidable motion caused by respiration and heartbeat from the patient.